This invention relates to strain sensing and more especially to a strain sensor, apparatus for use with and a method of operating a strain sensor for sensing structural health and load monitoring.
Structural health and load monitoring of structures such as bridges and buildings is well known. Typically such systems measure the tensile or compressive strain within the structure, that is the change of length (extension or contraction) relative to the original length, which is indicative of the loading of the structure. Such information can be used in assessing damage and warning of impending weakness in the structural integrity of structures such as aircraft, space platforms, marine vessels, bridges and other structures as well as in their engineering design.
To measure strain within such structures it is known to use a strain sensor. Early strain sensors relied on a change in electrical resistance with strain and typically comprised four terminal devices in which two terminals were used to apply electrical current to the device and the other two for accurately sensing the potential difference developed across it. A particular disadvantage of such electrical resistance sensors is that when it is required to measure strain at a large number of points, as would be the case in structural monitoring of structures such as bridges and buildings, such sensors require a very large number of electrical connections, making them cumbersome and prone to electrical failure.
More recently optical fibre strain sensors have been proposed which overcome a number of the problems of electrical resistance sensors. Optical fibre strain sensors comprise an optical fibre containing a number of components which are responsive to applied strain. Such components can comprise birefringent elements, micro-bends, Fabry-Perot resonators or intra-core Bragg gratings. In the case of the latter which are often termed fibre Bragg gratings, each Bragg grating which itself constitutes a respective strain sensor, reflects light at a characteristic wavelength which is determined by the pitch of the grating. This characteristic wavelength will change if the optical fibre is subjected to tensile or compressional strain which affects the pitch of the grating. Strain is measured by measuring a change in the characteristic wavelength of each grating. By providing a number of gratings along the length of the fibre, each of which reflects light at a different characteristic wavelength, it is possible to measure strain at a number of different points along the optical fibre.
Optical fibre strain sensors offer a number of advantages compared to electrical strain sensing techniques, making them attractive for structural health monitoring applications. For example, the Bragg grating characteristic wavelength is a linear function of change in grating pitch; fibre Bragg gratings are inherently wavelength encoded and consequently problems of intensity magnitude variation are eliminated, being fully integrated within the optical fibre eliminates any point of mechanical weakness, they are immune to electro-magnetic interference (EMI), are lightweight, resistant to corrosion and fatigue, inherently safe in that they cannot initiate fires or explosions and are compatible with fibre reinforced materials. In relation to the latter their compatibility has lead to the emergence of so-called xe2x80x9csmart structuresxe2x80x9d which structurally integrate optical fibre sensors thereby enabling continual monitoring of the internal strain of the structure and/or any load to which it is subjected.
Whilst optical fibre strain sensing is found to be effective the inventors have appreciated that it suffers from certain limitations. Fibre Bragg gratings can be addressed in the wavelength, time and space domains. The number of fibre Bragg grating sensors that can be integrated into a single fibre and addressed by wavelength multiplexing is limited which is a consequence of the limited spectral range of the optical sources which are used to operate such sensor systems. Typically, the spectral range of the currently available optical sources is 30 to 40 nm and it is usually required to be able to measure strains in the region of 3,000 to 5000xcexcxcex5 (that is a 0.3%-0.5% mechanical extension/contraction) which corresponds to a change in the characteristic wavelength of between 3 to 5 nm. In order to effectively operate a number of Bragg grating sensors within a single optical fibre it is necessary to dedicate a well defined wavelength range to each sensor to ensure that at its maximum wavelength change the characteristic wavelength of any given sensor cannot intrude upon the wavelength range of the sensor in the adjacent wavelength band since, under these conditions, it is impossible to discriminate between light reflected from the two sensors. As a result the number of gratings that can be incorporated in a single fibre is limited.
The present invention has arisen in an endeavour to overcome at least in part the limitations of the known strain sensing arrangements.
According to the present invention a strain sensor comprises an optical waveguide having a plurality of reflecting structures along its length, wherein each structure reflects light at a different characteristic wavelength which changes in dependence on a change of physical length of at least part of the reflecting structure; characterised in that the reflectivity of reflecting structures which reflect at characteristic wavelengths which are adjacent to each other are configured to be different such that the intensity of light reflected from adjacent structures can be used to discriminate between them. Since discrimination between the reflection characteristics of structures which are adjacent in wavelength is based on the relative magnitude of their reflectivities, this allows reflecting structures to have overlapping wavelength bands thereby enabling more reflecting structures to be incorporated within an optical waveguide for a given optical spectral range.
By securing the regions of the optical waveguide which include the reflecting structures, to an object, any change in length of the object will cause a change in the length of the reflecting structure which will be detected as a change in the characteristic wavelength. Furthermore, if these regions of the optical waveguide are placed in thermal contact with an object, any change in temperature will cause a change in the physical length of the reflecting structure which will be detected as a change in characteristic wavelength and the strain sensor of the present invention thus acts as an effective temperature sensor. It will be appreciated that in both measuring strain and temperature the strain sensor measures a change in the length of at least a part of the reflecting structure, that is it measures an internal strain of the sensor. In the context of the present invention the term strain sensor is intended to be construed broadly as a sensor which relies on a change in length and should not be restricted to a sensor which is for measuring strain in an object to which it is attached.
Advantageously the reflecting structures which reflect at adjacent wavelengths are configured such that one structure reflects light at one characteristic wavelength and the structure adjacent in wavelength is selected to reflect light at two characteristic wavelengths. Preferably the reflecting structure which reflects light at two characteristic wavelengths is configured such that the two wavelengths are separated by at least the width of the reflection characteristic of the structure which reflects at the adjacent wavelength. Such an arrangement is particularly advantageous since at least one of the pair of characteristic wavelengths always remains resolvable and therefore discrimination between the reflecting structures is possible.
Most conveniently the optical waveguide comprises an optical fibre and preferably the or each reflecting structure comprises a grating structure, most preferably a Bragg grating, in which a change in the characteristic wavelength is in consequence of a change in the pitch of the grating. In a preferred implementation the optical fibre includes a photo refractive dopant, such as for example a silica optical fibre doped with germanium oxide, and the or each grating structure is optically written into the fibre core by, for example, exposing the fibre to ultra-violet (UV) holographic projection. In such a case the spacing of the fringes of the holographic projection determines the pitch and hence the characteristic wavelength of the grating and the intensity of the UV light determines the reflectivity at the characteristic wavelength.
According to a second aspect of the invention an apparatus for measuring strain comprises a strain sensor described above; a light source operable to apply light to the waveguide of the sensor, said light having a wavelength range which covers at least the range of wavelengths over which the reflecting structures reflect and detector means for determining the change of characteristic wavelength at which each reflecting structure reflects light, said changes being indicative of a change in length of at least a part of the respective reflecting structure.
Preferably the detector means determines the changes in characteristic wavelength by measuring the wavelengths at which the sensor reflects light. Since the strain sensor only reflects light at various characteristic wavelengths, any light which is not reflected will pass along the optical waveguide substantially unattenuated. As a result, at the far end of the waveguide the changes in wavelength will appear as a change in attenuation of transmitted light. In an alternative arrangement the detector means measures light transmitted by the sensor and determines the changes by measuring the changes in wavelength at which light transmission is attenuated.
In a particularly preferred form of apparatus the detector means further comprises means for utilising the relative magnitude of the intensity of the reflected light or the relative magnitude of the intensity at which light transmission is attenuated to discriminate between reflecting structures which are adjacent in wavelength.
According to yet a further aspect of the invention a method of measuring strain comprises providing a strain sensor described above; applying light to the waveguide of the sensor, said light having a wavelength range which covers at least the range of wavelengths over which the reflecting structure reflects light, and detecting any change in the characteristic wavelength at which the reflecting structures reflect light. Preferably the changes in characteristic wavelength are detected by measuring the wavelengths at which the sensor reflects light.
Alternatively the changes in characteristic wavelength can be detected by measuring the wavelengths at which the transmission of light through the sensor is attenuated.
Preferably the method further comprises detecting the relative magnitude of the intensity of reflected light or the relative magnitude of the intensity at which transmitted light is attenuated to discriminate between reflecting structures which are adjacent in wavelength.
When it is desired to measure strain within an object the method further comprises securing a part of the waveguide having at least a part of one of the reflecting structures to the object such that a change in the physical length of the object causes a change in the physical length of the reflecting structure. Alternatively, or in addition, when it is desired to measure the temperature of an object, the method further comprises placing a part of the waveguide having at least a part of one of the reflecting structures in thermal contact with the object such that a change in the temperature of the object causes a change in the physical length of at least a part of the reflecting structure.